Applicant's prior international patent application No. PCT/AU81/00188 (International Publication No. WO81/01109) discloses a simple microfiche constituting an information storage medium of the kind in question That application outlines a technique for compensating for fractional misregistration between detector elements of the read head and optically machine readable data entries in the fiche, and also suggests compensation for integral or larger scale misregistration by utilizing characteristic marker patterns among the data entries in the frames.
It is known to synchronize read heads with rows of data entries by means of accurate synchronization markers at the end of each row. In the most basic case, such markers comprise perforations which register with sprocket teeth, but machine readable markers are also employed. It is further known to provide, for synchronization purposes, markers of specific shape among data entries, or characteristic opaque or transparent zones.
The significance of applicant's aforementioned international patent application is that it renders practicable the storage of information on microfiche in optically readable binary format. It is an objective of the present invention to provide a convenient arrangement of synchronization markers which occupies a minimum of possible information storage space and which performs substantially all the principal synchronization functions required. It is a further objective to provide a method of reading information from a storage medium of the kind described.
Even once synchronization has been initially achieved, local faults may arise in the regular position indicators or markers by which synchronization is sustained. Many devices currently exist which need to determine the position of a read head relative to data, but most such devices are read/write in that they can record data as well as read it. Any fault during the writing process may result in mispositioned data and the reading process needs to recognize this possibility. Hence it is common for each position determination to override all previous determinations in an absolute fashion, or if any note of previous determinations is made, they serve only to alert the reading system to the presence of an error.
In the case of magnetic discs, for example, the synchronization pattern along the track (whether before the sector number data or on the servo surface) must normally be complete and accurate for the sector to be accessible. Any fault in the pattern generally results in the sector being unusable.
Similarly the next frame on magnetic tape drives must be observed within a specified period since the last or an error is reported, so that a systematic error in frame positioning such as an incorrect density cannot in general be accommodated.
Such position determination processes as used in the conventional computer context may well be best suited to read/write media. When applied to a read only medium whose production is carefully controlled, such as the optically readable media with which this invention is primarily concerned, they suffer excessive sensitivity to faults arising from spot obliteration such as by dust, medium nonuniformity etc. Reverting to the example above, a single bit fault in the synchronization pattern on a magnetic disc is sufficient to render the entire sector which follows unusable. When a medium is subject to a high number of such spot faults, a new approach needs to be taken to ensure accurate position determination so that local faults or "spot errors" in synchronization markers or patterns, or other position indicators, are distinguishable from more serious system deficiencies and do not render whole segments of data unread or unreadable.